Display

ABSTRACT

An aspect of the invention provides a method of recording an image, comprising: providing a display cell having a first cell wall and a second cell wall opposed to the first cell wall, the cell walls enclosing a layer of a display medium comprising a liquid crystal material having finely-divided pigment particles dispersed therein, each cell wall including an electrode for applying an electric field across the display medium; applying via the electrodes a first electric field of a first polarity and of sufficient magnitude and duration to cause the particles to migrate and accumulate at the first cell wall; illuminating at least some of the particles with an image to be recorded; and applying via the electrodes a second electric field of opposite polarity to the first polarity and of sufficient magnitude and duration to cause some but not all particles to migrate from the first cell wall so as to produce a recorded image.

This application claims priority to co-pending United Kingdom patentapplication number 0700187.8 filed on Jan. 5, 2007, which is entitled“DISPLAY” the disclosure of which is incorporated herein by reference.

The present invention relates to a display, and to a method of recordingan image on the display and a method of erasing the image.

Practical paper-like displays require high brightness and bistability,preferably with low power consumption and simple construction. Manytechnologies have been employed in an attempt to develop such displays,including electrophoretic devices in which an image is formed bypatterned electrodes which define pixels. An example of such a device isdescribed in US20050094087A1.

According to a first aspect of the invention there is provided a methodof recording an image, comprising:

-   -   providing a display cell having a first cell wall and a second        cell wall opposed to the first cell wall, the cell walls        enclosing a layer of a display medium comprising a liquid        crystal material having finely-divided pigment particles        dispersed therein, each cell wall including an electrode for        applying an electric field across the display medium;    -   applying via the electrodes a first electric field of a first        polarity and of sufficient magnitude and duration to cause the        particles to migrate and accumulate at the first cell wall;    -   illuminating at least some of the particles with an image to be        recorded; and    -   applying via the electrodes a second electric field of opposite        polarity to the first polarity and of sufficient magnitude and        duration to cause some but not all particles to migrate from the        first cell wall so as to produce a recorded image.

The invention provide a photoaddressable display device withbistability. The device provides a display with lower power consumptionthan many conventional displays. The device may be of simpleconstruction and can produce an image of high contrast and brightness,making it potentially suitable for use as a paper-like display.

I have found that, surprisingly, the behaviour of the particles under anelectric field differs according to the extent to which the particleshave been illuminated by light. Without wishing to be bound by theory, Ibelieve that the illumination locally reduces electrostaticstabilisation parameters of the display medium, enablingoptically-controlled spatial modulation of the electrophoretic effectand bistable recording of optical images. I theorize thatphotosensitivity and charge generation are provided by the intrinsicfeatures of the LC mixture with the pigment particles, due tointeraction of LC molecules with the pigment surface, when the LCmixture when placed between electrodes.

In one embodiment, the particles which migrate from the first cell wallunder the second electric field accumulate at the second cell wall.Under this condition, the display cell displays a positive image whenviewed through one cell wall, and a negative image when viewed throughthe other.

The term “light” is used herein to refer to visible light, and also toother wavelengths of light which produce the desired effect, for exampleultraviolet light.

The image remains stable until erased. Erasure may be achieved byapplying an electric field of opposite polarity to the first polarityand of sufficient magnitude to cause substantially all the pigmentparticles to migrate from the first cell wall. Alternatively, erasuremay be achieved by applying an alternating electric field of sufficientmagnitude and duration to cause substantially all the pigment particlesto migrate from the first cell wall.

It will be understood that reference to an electric field of aparticular polarity does not exclude a possible AC component, providingthat the overall polarity is sufficient to effect the desired migrationof particles.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example, withreference to the following drawings in which:

FIGS. 1A and 1B are a schematic views of a display device in accordancewith an aspect of the invention;

FIG. 2 shows a display cell in accordance with an embodiment of theinvention;

FIGS. 3A and 3B show an experimental display cell in accordance with anembodiment of the invention before and after having a light imagerecorded thereon;

FIGS. 4A, 4B, and 4C show photomicrographs of regions of the displaycell of FIG. 3;

FIGS. 5A and 5B show an experimental display cell in accordance with anembodiment of the invention after having a light image recorded thereon;under light illumination on front and rear sides;

FIG. 6 is a schematic diagram of an experimental setup for investigationoptical recording on display cell;

FIG. 7 shows an oscilloscope picture, displaying the optical response ofa display cell with a black back screen under an applied combination ofan electrical signal and microscope light (the upper curve displays theoptical response and the lower curve the applied electrical signal); and

FIG. 8 shows an oscilloscope picture, displaying the optical response ofthe display cell with a mirror on its back side under appliedcombination of an electrical signal and microscope light (the uppercurve displays optical response and the lower curve applied electricalsignal).

DETAILED DESCRIPTION

The display device 2 of FIG. 1 comprises a display cell 4 and in imagesource 20 for directing a light image onto the display cell 4. Thedisplay cell 4 comprises a first cell wall 6 and an opposed second cellwall 8. The cell walls may be formed from glass or plastics materialsknown per se in the art of manufacturing liquid crystal displays. Eachcell wall 6,8 has a corresponding electrode 18,16 on an inner surfacethereof. In this embodiment each electrode 16,18 covers substantiallyall of the area of the cell wall on which it is provided.

The cell walls 6,8 enclose a layer of a display medium 10, whichcomprises a liquid crystal material 12 (in this example, a nematicliquid crystal material) having finely-divided pigment particles 14dispersed therein. In this example, the liquid crystal material 12 has ablack dye dissolved therein. The dye may be pleochroic ornon-pleochroic, and simply functions to provide enhanced contrast to thepigment particles 30. The liquid crystal material 12 may have positiveor negative dielectric anisotropy.

The pigment particles in this example are titanium dioxide (TiO₂) whichprovides a bright white image in reflective mode. The particlespreferably have sizes less than 1000 nm, for example in the range100-500 nm. The high refractive index (n=2.72) and size of the TiO₂particles allow a direct observation of the electrophoretic effect whichresults in the collection of particles 14 close to the selected side ofthe display cell 4 under a suitable polarity of an applied voltage.

The image source 20 in this embodiment comprises a light source 22, alens 24 and a mask 26. Light from the light source 22 is focused throughthe lens 24 and mask 26 to produce a light image which is directed ontothe second cell wall 8 as shown in FIG. 1A. It will be understood thatother image sources may be used, for example a light-emitting displaysuch as a CRT, plasma panel, back-lit LCD panel or any other suitableimage source well known to those skilled in the display arts.

Initially, the pigment particles 14 are caused to collect close to thefirst cell wall 6 by application of a first electric field of a firstpolarity via the electrodes 16,18. The voltage to achieve this will varydepending on the types of components in the display medium 10, but willtypically be in the range 80-120 V, notably 100-110 V. This initialstate, with pigment particles 14 collected close to the first cell wall6 is illustrated in FIG. 1A.

The display cell 4 is then exposed with a light image from the imagesource 20, in this embodiment via the second cell wall 8. After thislight exposure, a second electric field of opposite polarity to thefirst polarity is applied as a pulse sufficient to cause some, but notall, of the particles 14 to migrate from the first cell wall 6, therebycreating an image 28. Suitable pulse voltages will vary depending on thespecific details of the system, but will typically be from 10-80 V,notably about 30-60 V, and 5-50 ms duration. The field strength willtypically be in the range 3-6 V/μm.

The image 28 comprises first picture elements 30 in which pigmentparticles 14 are aggregated at the first cell wall 6, and second pictureelements 32 in which pigment particles 14 are substantially not presentat the first cell wall 6. Each first picture element 30 stronglyreflects light when viewed through the first cell wall 6, and eachsecond picture element 32 absorbs some or all light when viewed throughthe first cell wall 6, providing substantial contrast between the firstand second picture elements. In the example shown in FIG. 1B, theparticles 14 in the second picture elements 32 have gathered close tothe second cell wall 8 so that if the display cell 4 is viewed throughthe second cell wall 8 the image appears as a negative of the image whenviewed through the first cell wall 6. A mixture of small first pictureelements 30 and small second picture elements 32 provides greyscalecapability when viewed from a suitable distance.

Because the displayed image 28 is determined by the light image from theimage source 20, first picture elements 30 and second picture elements32 may both be contained within a region where the electrodes 16,18overlap. In the present example, this region comprises substantially theentire area of the cell walls 6,8. It would of course be possible foreach cell wall to be provided with more than one electrode, for exampleas ‘row’ electrodes on the first cell wall 6 and ‘column’ electrodes onthe second cell wall 8; however such alternatives complicate the displaywithout conveying any benefit, so the exemplified display cell 4 with asingle electrode on each cell wall is preferred.

It will be understood that the image 28 may readily be reversed byreversing the polarities of the applied electric fields depending on thecell wall through which the display cell 4 is to be viewed.

The image 28 produced by the experimental display device 2 shown in FIG.1 is a simple spot. More complex images 28 can readily be produced byusing more complex masks, for example as illustrated in FIG. 2. Theimage 28 of FIG. 2 was produced using a display cell 4 with a spacing of10 μm. The display medium 10 comprised 70% LC ZLI4756/2 doped with ablack dye and 30% TiO₂ pigment WP10S.

The dielectric permittivity of the liquid crystal is believed to bechanged in a region where the particles are concentrated, because theliquid crystal molecules will tend to be randomly aligned by localinteractions with the pigment particles; consequently the dielectricpermittivity will be low. Applying a voltage aligns the liquid crystalmolecules, switching the dielectric permittivity to a higher value, dueto which a large electrical dipole will be induced around the pigmentparticles.

Without wishing to be bound by theory, we believe that optical radiationwith an applied strong field generates locally in the illuminated areaselectrical charges which will reduce an electrical double layer aroundthe pigment particles 14, providing electrostatic stabilisation relativeto pigment particles in non-illuminated areas. Consequently, thispromotes aggregation of pigment particles in the illuminated areas.Because of this, the applied pulse with reversed polarity and sufficientamplitude forces motion of the pigment particles in the non-illuminatedareas (with a larger electrical double layer) towards the second cellwall 8 (FIG. 1B). This effect is believed to produce the spatialmodulation of the electrophoretic effect and provide optically observeddifferences between the illuminated and non-illuminated areas.

The effect is bistable and both switched states are stable after removalof the voltage. Erasing of the image 28 is achieved by application of anelectrical pulse with a higher amplitude (about 6-10 V/μm or higher),and alternatively by applying an alternating voltage.

The aggregation of pigment particles 14 in the illuminated area wastested in transmissive mode in the 5 micron display cell 4 (FIG. 3), inwhich the display medium comprised 80% LC ZLI4756/2 doped with a blackdye and 20% TiO₂ pigment WP10S. When an alternating voltage is appliedacross the display cell 4, which alternation is shorter than the drifttime for pigments to drift from one cell wall to the other, the pigmentparticles do not collect close to the cell walls and will be uniformlydistributed in the volume of the cell. In this case the display cell 4scatters the transmitted light and the texture looks dart (FIG. 3A).Illumination induces the aggregation of the pigment particles 14 andconsequently the illuminated area forms a transparent image 28 (FIG.3B). This effect is clearly illustrated in the photomicrographs of FIG.4. FIG. 4A shows the texture before illumination and FIG. 4B shows theboundary between illuminated and non-illuminated areas. FIG. 4C showsthe texture inside the illuminated area, where large domains ofaggregated pigment particles are observed.

When such a cell is observed under light impinging on the front of thecell (FIG. 5A), the scattered texture reflects light and the writtentransparent spot substantially does not reflect light. Turning the cellover and illuminating its rear side (FIG. 5B) confirms that that thetransparent texture of a written spot extends through the whole cell.

In this experiment, the light source 22 was a white light box with 150Watt lamp, the output of which was fed through a fibre-optic cable withan output diameter of about 4 mm. The lamp provided controllableilluminance in the range 200-10000 I×. Another experiment was carriedout using a microscope, with the experimental cell placed on microscopestage. As an optical source was used a microscope 20 Watt lamp, thelight from which was focused to a spot about 2 mm in diameter. FIG. 6shows the optics, in which the same light source is used for an opticalwriting and an optical detecting signal. In this case the light from thelamp is deflected by a prism to the cell. The transmitted light isfocused by a lens into the spot about 2 mm. Behind the cell was placed ablack screen (light absorber film). The reflected light from the cellpasses back through the lens and the prism, and is detected by aphotosensor, which is connected to an oscilloscope, displayingmodulation of reflected light. To the cell are applied electrical pulsesfrom the generator.

Bursts of bipolar pulses (FIG. 7 ) were applied to the cell. One of thebursts contains 2 ms bipolar pulses with repetition frequency 200 Hz,and a second burst contains 20 ms bipolar pulses with increasingamplitude and a repetition frequency of 50 Hz. As follows from theoscilloscope picture the intensity of the reflected light is decreasedbeginning from some amplitude of applied voltage. This indicates that inthe specified case for the amplitude about 100 V, the light spotproduces a writing effect, and in the spotted area the texture becomestransparent. Due to this the light will be absorbed by the black layerand consequently the optical response is decreased. The next pulsepocket has less amplitude and a pulse duration of 2 ms, which is lessthen the time drift across the cell. Accordingly, this pulse pocketproduces a transition of the written texture to the scattered texturewith uniform distribution of pigments in the cell volume andconsequently the intensity of reflected light is increased. Theexperiment shows that the magnitude of the electrical field strength isimportant for the optical effect. For the specified case the microscopelight produces an optical effect when the amplitude reaches a valueabout 100 V for a pulse duration of about 20 ms.

The optically written state has a quite good transparent texture for acell with 5 μm thickness. For a further experiment, the optical schemeshown in FIG. 6 was used but with the black layer replaced by a mirror.FIG. 8 shows modulation of reflected light from the cell under twobursts of applied bipolar pulses: one with amplitude more than 100 V andlength 5 ms with repetition frequency of 80 Hz, and the other withamplitude 60 V and length 2 ms with repetition frequency 200 Hz. Asfollows from the pictures the burst with high amplitude produces a highintensity of reflected light. This result suggests that the spotted areabecomes transparent and light passes through this LC texture andreflects back from the mirror with high intensity. The burst with loweramplitude does not produce this optical effect and only uniformlydisperses the pigments in the whole cell. Consequently this statereflects less light from the same spotted area. A quite big differencebetween intensities of reflected light indicates that the opticallyinduced state has a good transparency.

If we suppose that the lamp's light provides irradiance 0.15 W (˜100Im), the light energy providing optical writing during an appliedelectrical pulse 20 ms will be around 0,15 J/s×2.10⁻² s≈0.003 J. Theexperiment shows that this effect gives a possibility to modulate lightby controlling the transparency of the electrophoretic cell by applyinga suitable combination of electrical and optical signals.

Experimentally the display cells 4 have been constructed from glass orplastic cell walls with transparent ITO electrodes. Switching wasobtainable using uncoated electrodes. However, for better switchinguniformity, both of the cell walls over the ITO electrodes were coveredby a thin layer providing local homeotropic alignment to the liquidcrystal material 12. Polymer beads (not shown) were used to control thespacing between the cell walls. The size of the polymer beads rangedfrom 5-20 μm. The following commercial nematic liquid crystal materialsfrom Merck were used: E7, ZLI2293, MLC6681, MLC6650, MLC6204-000,MLC6436-000 doped by blue and magenta dyes or by CuO black pigment;dye-doped nematics ZLI3752, ZLI4756/2 (all with positive dielectricanisotropy) and ZLI4788, MDA-03-4518, MDA-03-4517, dye-doped ZLI6092(all with negative dielectric anisotropy). These were doped with TiO₂particles. The TiO₂ particles used were: R700, R900, R931, R106 withsizes 300-400 nm (DuPont) and WP10S, RP10S with sizes 200-300 nm(Catalysts & Chemicals Ind. Co. Ltd). The concentration (by weight) ofthe particles in the display medium 10 varied from 5-50%. To the displaycell 4 were applied unipolar pulses with amplitude 10-80 V, and pulseduration 5-50 ms.

The pure nematic LCs with transparent pigments Hosteperm Blue B2G-D,Hosteperm Magenta

E02, Novoperm Yellow 4G (from Clariant) were also used.

The invention therefore provides optical addressing in anelectrophoretic device, which gives the possibility of reversiblephotographic recording. We believe that the combination of electricalfield and optical radiation locally reduces electrostatic stabilisationparameters of the fluids with added pigment particles, enablingoptically controlled spatial modulation of the electrophoretic effect.

1. A method of recording an image, comprising: providing a display cellhaving a first cell wall and a second cell wall opposed to the firstcell wall, the cell walls enclosing a layer of a display mediumcomprising a liquid crystal material having finely-divided pigmentparticles dispersed therein, each cell wall including an electrode forapplying an electric field across the display medium; applying via theelectrodes a first electric field of a first polarity and of sufficientmagnitude and duration to cause the particles to migrate and accumulateat the first cell wall; illuminating at least some of the particles withan image to be recorded; and applying via the electrodes a secondelectric field of opposite polarity to the first polarity and ofsufficient magnitude and duration to cause some but not all particles tomigrate from the first cell wall so as to produce a recorded image.
 2. Amethod according to claim 1, wherein the first electric field has afield strength greater than about 6 V/μm.
 3. A method according to claim1, wherein the second electric field has a field strength in the range3-6 V/μm.
 4. A method according to claim 1, wherein the application ofthe second electric field causes particles which migrate from the firstcell wall to accumulate at the second cell wall.
 5. A method of erasingan image produced by the method of claim 1, comprising applying anelectric field of opposite polarity to the first polarity and ofsufficient magnitude to cause substantially all the pigment particles tomigrate from the first cell wall.
 6. A method of erasing an imageproduced by the method of claim 1, comprising applying an alternatingelectric field of sufficient magnitude and duration to causesubstantially all the pigment particles to migrate from the first cellwall.